System and method for transferring between boiler-turbine plant control modes

ABSTRACT

A boiler-steam turbine plant control system for transferring between boiler-follow, turbine-follow, and coordinated operating modes, without any change in the output of the turbine or the firing rate of the boiler, is disclosed. A feedforward load demand signal controls both the turbine and boiler in parallel. In the turbine-follow mode, a feedback loop trims the feedforward signal to the turbine only. In the boiler-follow mode, a feedback loop trims the feedforward signal to the boiler only. In the coordinated mode, feedback loops trim the feedforward signal to both the turbine and boiler. In response to initiating a transfer to either the boiler or turbine-follow mode, the load demand signals which are controlling the boiler and turbine are held at their pre-transfer value; and the feedforward signal is modified to equal the pre-transfer value of the trimmed feedforward signal. A signal, which is equal in value to the pre-transfer value of the feedforward signal less the value of the trimmed feedforward signal is generated in the output of the feedback loop which is to be placed in service as a result of the transfer. To transfer to the coordinated mode, the feedforward signal is modified to equal a signal representative of the actual power output of the plant. The turbine feedback loop generates a signal representative of the pre-transfer turbine demand signal less the modified feedforward signal. The boiler feedback loop generates a signal representative of the pre-transfer boiler demand signal less the modified feedforward signal.

United States Patent [1 Stem [ Dec. 9, 1975 SYSTEM AND METHOD FORTRANSFERRING BETWEEN BOILER-TURBINE PLANT CONTROL MODES v [75] Inventor:Louis P. Stern, Pittsburgh, Pa.

[73] Assignees Westinghouse Electric Corporation,

Pittsburgh, Pa.

[22] Filed: Mar. 7, 1975 [21] Appl. No.: 556,363

[52] US. Cl 235/15l.2l; 290/40 R; 60/646 [51] Int. Cl. F01D 17/02; GOSB15/02 [58] Field of Search 235/151.21;,60/646; 290/40 R, 40 A-C, 40 F;340/1725; 415/17; 444/1 [5 6] References Cited UNITED STATES PATENTS3,552,872 1/1971 Giras et a1. 415/17 3,555,251 1/1971 Shavit 235/1513,561,216 2/1971 Moore, Jr... 60/73 3,564,273 2/1971 Cockrell 415/17 X3,588,265 6/1971 Berry 415/17 X Primary ExaminerEdward J. Wise Attorney,Agent, or FirmH. W. Patterson [57] ABSTRACT A boiler-steam turbine plantcontrol system for trans- CPB J- PLANT MODE TRANSFER CIRCUITRY ferringbetween boiler-follow, turbine-follow, and coordinated operating modes,without any change in the output of the turbine or the firing rate ofthe boiler, is disclosed. A feedforward load demand signal controls boththe turbine and boiler in parallel. In the turbinefollow mode, afeedback loop trims the feedforward signal to the turbine only. In theboiler-follow mode, a feedback loop trims the feedforward signal to theboiler only. In the coordinated mode, feedback loops trim thefeedforward signal to-both the turbine and boiler. In response toinitiating a transfer to either the boiler or turbine-follow mode, theload demand signals which are controlling the boiler and turbine areheld at their pre-transfer value; and the feedforward signal is modifiedto equal the pre-transfer value of the trimmed feedforward signal. 'Asignal, which is equal in value to the pre-transfer value of thefeedforward signal less the value of the trimmed feedforward signal isgenerated in the output of the feedback loop which is to be placed inservice as a result of the transfer.

To transfer to the coordinated mode, the feedforward signal is modifiedto equal a signal representative of the actual power output of theplant. The turbine feedback loop generates a signal representative ofthe pre-transfer turbine demand signal less the modified feedforwardsignal. The boiler feedback loop generates a signal representative ofthe pro-transfer boiler demand signal less the modified feedforwardsignal.

15 Claims, 9 Drawing Figures US. Patent Dec. 9, 1975 Sheet 2 of53,925,645

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BUFFER US. Patent Dec. 9, 1975 Sheet 4 of5 3,925,645

LDC FIG.7 ANALOG OUTPUT A J'A'A' M RAISE INHIBIT LOWER INHIBW 37 DN upF|G.8

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D/A CONVERTER 152 AUTO 100% o o A OUTPUT OUTPUT O V o MAN 0 SYSTEMANDMEIHQD FOR T ANSEER I G BETWEEN BOILER-TURBINE PLANT CONTROL PE I'BACKGROUND OF THE INVENTION The present invention relates tocontrolsystems for power plants; and more particularly to a system fortransferring rapidly between operating modes without a change in theoutput of the turbine or' the firing rate of r the boiler.

The improved control of power generationis accomplishedefficiently byachieving a close coordination of the boiler and turbine controls. This:control is achieved by a plant master control unit or load demandcomputer, whichgenerates a feedforward load demand signal that isconnected in parallel to a turbine master control apparatus foroperating the steam inlet valves to a predetermined position inaccordancewith the valve of the feedforward signal, and to a boilermaster control apparatus for controlling the fuel, air, and water to theboiler at a predetermined rate in accordance with the value of thefeedforward signal. The feedforward demand signal from the load demandcomputer minimizes interaction between the turbine and boiler, andextracts the best possible dynamic response ofthe plant. a v Inaddition, under normal steady-state operating conditions, the loaddemand signal to the turbine and the boiler master control is trimmed byan error signal from an analog feedbackloop, in response to detection ofthrottle pressure error and megawatt pressure error. In particular, themeasured throttle pressure is compared to a reference level determinedby a set point to pro vide the throttle pressure error signaLwhich isintegrated and applied to the turbine control. The output'of the turbineis measured and compared with a reference to provide megawatt error,which is integrated and applied to the boiler control. v

A typical modern plant 'control system provides for several differentmodes of operation for various plant conditions. For example,. in theabove described mode, which may be termed a coordinated mode, a throttlepressure error feedback loop and a megawatt error feedback loop is inservice. One feedback loop trims .the load demand signal to the boilercontrol with the megawatt error signal; and the other feedback looptrims the turbine load demand signal to the turbine control with thethrottle pressure errorsignali In this mode, an increasing load demandresults in the throttle pressure and megawatt generation to be' low.Thus, the .throttle pressure set point is increased by an amountproportional to generator error; and atthe same time the firing rate ofthe boiler is increased, the'throttle pressure is lowered momentarilyasthe governor valves are opened to permit greater steam flow.

, -;.In the event of certain abnormal conditions, the systemcan betransferred either automatically or manually to either a boiler-followor a turbine-follow mode.

If an unusual turbine condition develops, the system is transferred tothe boiler-follow mode wherein the feedforward signal from the plantunit master or load dewherein the feedforward signal sets the boiler tooperate according to predetermined limits, and the feedforward signal istrimmed by the turbine feedback loop to adjust the governor valves tomaintain a substantially constant pressure.

In the event that there is no error signal on the feedback loop inservice at the time of the transfer, the

"megawatt output of the plant, of course is not affected by suchtransfer. However, such a situation is not ordinarily the case. Usually,the plant is operating with a positive or negative error on the outputof the feedback loop. Although variations in the BTU capacity of fueloil in a boiler creates such a condition, the problem is aggravated inplants where coal is used, to the extent that the feedback error signalcan vary up to fifteen percent of the feedforward load demand from onegrade of coal to another, thus, when one of the feedback loops is takenout of service, the error signal is removed, which in turn, causes themegawatt output of the generator firing rate of the boiler, or positionof the turbine valves to change abruptly. Of course, any changeoccasioned by such transfer is undesirable; and an abrupt change isparticularly undesirable.

Heretofore, a transfer between operating modes such as from theturbine-follow mode, to the boiler-follow mode, for example, waseffected without an abrupt change in the output of the turbinebygradually eliminating, or in other words decaying, the error signal fromthe feedback controller in the analog loop of the turbine to zero. Thethrottle pressure error feedback loop is connected to the boiler controlat zero error;

did not eliminate a change in the megawatt output of the turbine, andthe time required to effect such a transfer, depending on the mode towhich the transfer is made, could consume as such as 10 minutes, forexample.

In view of the increasing power demands placed upon large generatingsystems, it becomes increasingly important to provide a reliable poweroutput without a change, be it ever so gradual, even for a period in theneighborhood of ten minutes. The problem in such transfers is not onlyone of correcting for the feedback signal of the analog loop to beremoved from service; but it is also a problem concerned with the effecton the system of the analog feedback loop being placed in service. Thus,it is desirable to be able to effect a transfer between the variousoperating modes, rapidly, such as SUMMARY OF THE INVENTION Broadly, thepresent invention relates to a system and method of transferring fromone plant control mode to anotherin a boiler-steam turbine power plantwherein a feedforward signal representative of the desired plant load isgenerated to control in parallel the boiler portion of the plantand thesteam turbine portion of the plant in each of the modes. The trimmed anduntrimmed load control signals to both the boiler and the turbine areheld at their pre-transfer value in response to the initiation of atransfer.

The feedforward signal is modified to represent a selected valuedepending on the operating mode being entered. If one of the controlapparatus is to be operated with no feedback loop in service, theselected value is the pre-transfer value 'of its trimmed input.

For each of the loops to be put in service as a result of the transfer,a feedback signal is generated so that the input signal to itsrespective boiler and turbine control apparatus is representative of theboiler and turbine control apparatus input signal which is held at itspre-transfer value. Upon completion of the transfer, the boiler andmaster control are permitted to respond to their respective trimmed anduntrimmed signals in the transferred mode.

In a specific aspect, the system includes a load demand computer whichgenerates feedforward signalsin parallel to the turbine and boilercontrol apparatus. A feedback loop is provided for the turbine controlapparatus which trims the feedforward signal tocorrect for any deviationin throttle pressure. A feedback loop is provided for the boiler controlapparatus to correct for any deviation in throttle pressure; and afeedback loop is provided for the boiler control apparatus whichoperates in conjunctionwith the feedback loop for the turbine controlapparatus to correct for throttle pressure and megawatt error. Each oneof the feedback loops includes a proportional plus integral controller,and its error signal is summed with the feedforward signal to providethe trimmed signal. A manual/automatic transfer device for each mastercontrol apparatus receives the signal from the summing device andgenerates a signal corresponding to the summed signal for its associatedmaster control. In response to the initiation of a transfena digitalpulse of predetermined duration is generated, which governs eachmanual/automatic station to holdits output signal at its pre-transfervalue. The feedforward signal to each of the summing devices is modifiedto be representative of a selected pre-transfer value, which selectedvalue for a transfer where a particular control apparatus will have nofeedback loop in service is the pre-transfer value of the output of themanual/automatic station for such control apparatus. For the feedbackloops to be placed in service, the feedback controller is governed togenerate an output signal for input to each summing devicerepresentative of the pre-transfer value of the output of the manual-/automatic transfer station less the modified feedfor ward signal to thesumming device.

In transferring to the coordinated mode, for example, the feedforwardsignal is modified to be representative of the actual megawatt output ofthe plant. The turbine 4 or boiler feedback loops are each operated togenerate a signal which isrepresentative of the pre-transfer output ofits manual/automatic station less the modified feedforward signal. Theoutputs of the feedback loops are summed with the modified feedforwardsignal.

In response to termination of the digital pulse, the hold on the outputsignal of both manual/automatic stations is released, and the feedbackcontroller for the connected loop is permitted to change its signal tothe summing device in accordance with the error between the actualpressure or megawatts and the set point as the case may be,

IN THE DRAWINGS I selector foruse in the system of the invention;

FIG. 5 is a schematic diagram of one form of a proportional plusintegral feedback controller for use in the system of the invention;

FIG. 6 is a schematic diagram of one fonn of a summing device for use inthe system of the invention;

FIG. 7 is a schematic diagram of one form of a difference functiondevice foruse in the system of the invention;

FIG. 8 is a schematic diagram of one form of a manual/automatic stationfor use in the system of the invention; and

FIG. 9 is a schematic diagram of one form of a mode select logic circuitfor use in the system of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring specifically to FIG.1, there is shown schematically a general block diagram of an analog'plant control system wherein analog feedback error signals areselectively connected to be summed with a feedforward signal foroperating the turbine and boiler control in accordance with a selectedplant operating mode; and includes a block diagram of a system fortransferring between such operating modes with a change in the positionof the turbine inlet valves, boiler firing rate, or the megawatt outputof the turbine.

A load demand computer 10, which is shown in detail in FIG. 3 generatesa load demand signal on output 11 to the input of summing devices 12 and13, the details of which are shown in FIG. 6. The load demand computer10, may be any signal generating device which functions as disclosed,and is constructed to generate a feedforward signal, the value of whichis calculated to correspond to a desired load for the plant. A modeselect pushbutton BPB is operated toplace the plant in a boiler-followmode; a mode select pushbutton TPB is operated to place the plant in aturbine-follow mode; and a pushbutton CPB is operated to place the plantin a coordinated mode. In response to the operation of selected ones ofthe pushbutton BPB, TPB, and CPB, logic circuitry described inconnection with plant mode transfer circuitry 28 is employed to energizeselected ones of the outputs 14, 15 and 16, for

opening or closing respective relay contacts 17, 18 and 19, to connectand disconnect outputs 20, 21 and 22 of feedback controllers 23, 24 and25, for input to the associated summing devices 12 and 13. Although,conventional relays and contacts are shown for the sake of simplicity,it is understood that other type switching circuits may be used.

The load demand computer may also include a device 26 for increasing ordecreasing manually the value of the analog signal on its output -11.This, of course, in an actual planemay be also accomplishedautomatically. The load demand computer 10 also includes an input 27,which, when energized by a digital signal from the transfer circuitry28, the details of which are shown in FIG. 2, functions to modify thefeedforward signal on the output 11 to be equal to an analog signal oninput 30 from the transfer circuitry 28; or in other words the loaddemand computer 10 tracks the value of the signal on the input 30, whenthe input 27 is energized.

The feedforward signal on the output 11 of the load demand computer 10is connected in parallel to a turbine master control 33 and to a boilermaster control 36. The signal to the summing device 12, is conducted byway of output line 31 to the manual/automatic station 32; for input tothe turbine master control mechanism 33. The signal on the output 11,which is input to the summing device 13, is conducted by output 34 to amanual/automatic station 35 for input to the boiler master control 36.

It is understood that the analog summing devices 12 and 13 may be of thetype shown in FIG. 6, or any well known device that functions asdescribed herein. The manual/automatic stations 32 and 35 may be of thetype shown in detail in FIG. 8, which function to either increase ordecrease the value of an analog signal manually or automatically. Eachof the manual/automatic stations 32 and 35 also function to prevent themanuallautomatic stations 32 and 35, when in automatic, from respondingto a change in the value of a signal on their input lines 31 and 34 attimes when a digital signal is present on output 37 of the transfercircuitry 28. In other words, the presence of a signal on line 37prevents a change in the value of the signal on line 38 to the turbinecontrol mechanism 33, and on line 40 to the boiler control mechanism 36.

The turbine master control 33 may be of the type that is well known inthe art, and includes all apparatus necessary to position the steaminlet valves to the required position in accordance with the value ofthe signal on its input 38. The boiler master control 36 also may be ofany well-known type that functions to adjust the fuel air and feedwaterof the boiler to control its firing rate in accordance with the value onthe input signal 40.

The feedback controllers 23, 24 and 25 provide the output error signalsfor input to the summers 12 and 13 depending upon the particular controlmode in which the system is operated. Each of the controllers, thedetails of which are shown in FIG. 8 for the preferred embodiment of theinvention, is a proportional plus integral type controller whichproduces at its respective output 20, 21 an error signal, which trimsthe feedforward signal on output 11, for more accurately controlling theturbine and boiler master control mechanism 33 and 36. Associated witheach of the controllers 23 and 24 is a device TP for detecting theactual throttle pressure of the plant during operation, and a set pointinput SP for applying to its controller the desired throttle pressure.The signal from TP is compared with the set point signal from SP todetermine the value of the error signal at the output of its associatedcontroller 23 and 24. The feedback controller 25 detects the actualmegawatt output from the turbine by a detector MW, which is compared mmthe feedforward signal on the output 11 of the loaddemand computer 10 toprovide an error signal on output 22, which corresponds to thedifference between the desired megawatt output and the actual megawattoutput of the plant. The feedback controllers 23, 24 and 25 also includerespective analog inputs 41, 42 and 43, and respective digital inputs44, 45 and 46 from the plant load transfer circuitry 28. Each of thecontrollers function to generate at their respective outputs 20, 21 and22, a signal representative of the analog signal on the associatedinputs 41, 42 and 43 at times when a digital signal is present on therespective inputs 44, 45 or 46 from the plant load transfer circuitry28. Thus, in response to a digital signal on lines 44, 45 and 46, whichmay be termed track enable inputs, the associated feedback controllerconducts an analog signal on its respective output 20, 21, 22, which isrepresentative of the value of the analog signal on its associated input41, 42 and 43. This function may be referred to as the feedbackcontroller tracking a particular analog signal; and the purpose of whichis discussed in detail in connection with the description of FIG. 2.

When the plant is in the turbine-follow mode, the feedback controller 23is connected to the summing device 12 through the closed contact 17, andthe feedback controllers 24 and 25 are disconnected from the summingdevice 13 because of the open condition of the contacts 18 and 19. Thus,the feedforward signal on the output 11 of the load computer 10 governsthe boiler master control without correction; while any differencebetween the actual throttle pressure and the throttle pressure set pointis corrected by the error signal on the output 20 of the feedbackcontroller 23, which is summed with the feedforward signal on the output11 by the summing device 12. The output of the summing device 12conducts the corrected or trimmed feedforward signal by way of output31, to the manual- /automatic station 32, which in turn applies suchsignal to the turbine master control 33 by way of its output 38.Therefore, in the event of an error between the actual throttle pressureand the desired throttle pressure as indicated by the detector TP andset point SP, the turbine inlet valves are opened' or closed to agreater or lesser extent than commanded by the feedforward signal inorder to maintain the throttle pressure at the desired set point.

In the boiler-follow mode, the feedback controller 24 is connected tothe summing device 13 through closed contact 18, and the feedbackcontrollers 23 and 25 are disconnected from the respective summingdevice 12 and 13 because of the open condition of the contacts 17 and19. Thus, the feedforward signal on the output 11 controls the turbinevalves without correction; and the firing rate of the boiler isincreased or decreased to compensate for variations in the set pointpressure, by the feedback signal on the output 21 which is summed withthe feedforward signal by the summing device 13 to conduct a correctedor trimmed signal to the manual/automatic controller 35 for governingthe boiler firing rate through the boiler master control mechanism 36.

In the coordinated mode, the feedback controllers 23 and 25 areconnected to the summing devices 12 and 13 respectively through theclosed contact 17 and 19; and the feedback controller 24 is disconnectedfrom the summing device 13 because contact 18 is open. In this mode, thefeedforward signal on output 11 of the load demand computer is modifiedor trimmed by the throttle pressure error signal from the feedbackcontroller 23 to adjust the turbine inlet valves; and also, thefeedforward signal 11 to the summing device 13 is modified or trimmed bya megawatt error signal from the feedback controller 25 to adjust thefiring rate of the boiler in accordance with any deviation in thedesired megawatt output of the plant, as determined by the megawattdetector MW.

A transfer from one operating mode to another is initiated by theoperation of the appropriate pushbutton BPB, TPB and CPB to govern thetransfer circuitry 28 to hold the input signals to the turbine andboiler master controls 33 and 36 to the pre-transfer value to connectand disconnect the feedback controllers 23, 24 and 25, as previouslydiscussed; and to change the output of the load demand computer 10 inaccordance with analog input signals on lines 48 or 50 as the case maybe; all as described, in detail in connection with FIG. 2, in order thata transfer is effected without disturbing'the position of the turbineinlet valves, boiler firing or megawatt output of the plant.

Referring to FIG. 2, the plant mode transfer circuitry 28 is illustratedas being enclosed by the dashed lines. The portions of the circuitry ofFIG. 2 which are common with the circuitry of FIG. I bear similarreference numerals. It is to be understood that, although certain of thecomponents and circuitry are shown in the preferred embodiment as beingpart of the mode transfer circuitry 28, in actual practice, some or allcomponents may be structured within the feedback controllers 23, 24 and25, the load demand computer 10, the turbine master control 33, orboiler master control 36, for example.

The plant mode transfer circuitry 28 in the described embodimentincludes similarly constructed tracking signal selectors 51, 52, 53 and54, one form of which is shown in FIG. 4. However, each of the trackingsignal selectors may be any well-known analog device which responds to asignal to conduct a selected analog signal at its output. For example,the signal selector 51 functions to conduct an analog signal on itsinput 50 over its output 30 in response to a digital signal on its input55. Also, the presence of a signal on its input 36 operates the selector51 to conduct the analog signal on its input 48 over its output 30; andthe presence of a digital signal on its input 56C selects an analogsignal on line 57 for input to the LDC 10 over output line 30. Thus, thesignal on the output 11 of the load demand computer 10 is eitherincreased or decreased in accordance with the value of the signal online 30. The signal on the output 11 may be either increased ordecreased manually by manipulation of the mechanism 26, or eitherincreased or decreased by remote control apparatus of the plant referredto at block 58.

The tracking signal selector 52 functions to conduct an analog signal ofzero value on its output 41 to the feedback controller 23, asrepresented by its input 62 in response to a digital signal on its input60. The presence of a digital signal on input 61 causes the selector 52to conduct on line 41 the analog signal on its input 63. At times whenthe input 44, which may be termed a track enable input, of thecontroller 23 is energized, the controller tracks, or in other wordsgenerates at its output 21 an analog signal corresponding to the valueof the signal on line 41. When the track enable input 44 to thecontroller 23is deenergized, the output signal on line 21 corresponds tothe difference between the actual throttle pressure and the set pointthrottle pressure as determined by the signal from the elements TP andSP. Similarly, in response to a digital signal on input 64 of theselector 53, a signal of zero potential is input to the controller 24 togenerate a zero signal on its output 21 at times when track enable input45 to the controller 24 is energized. In response to a digital signal oninput 64 of the selector 53, the selector conducts an analog signal onits output 42, which in turn governs the controller 24 to track thevalue of the signal on input 66 of the selector 53 at times when thetrack enable input 45 is energized. At times when the input 45 of thecontroller 24 is deenergized, the controller generates at its output 21a signal corresponding to the difference between the set point signalrepresented by element SP and the actual throttle pressure detected byTP. Also, the signal selector 54 functions to conduct a zero signal fromits input 74 on its output 43 in response to a digital signal on itsinput 7 5; and, in response to a digital signal on its input 76, thesignal selector 54 conducts a signal representative of the value on itsinput 77, over its output 43. At times when the track enable input 46 isenergized the controller 25 generates an analog signal on its output 22corresponding to the signal on line 43 which signal corresponds toeither zero or the value of the signal on its input 77. At times whenthe track enable input 46 is deenergized, the controller 25 generates asignal corresponding to the difference between the value of the signalfrom the load demand computer 10 on its input 78 and the actual megawattoutput of the plant as detected by the detector MW. A component 76 whichis shown in detail in FIG. 7, may be a standard analog component whichconducts a signal at its output 63 which is equal to the valueof aninput signal on its line 68 less the value of an input signal on itsline 70. The designation (plus) and (minus) adjacent the input 68 and 70denote that the value of the signal on input 70 is algebraicallysubtracted from the value of the input signal on line 68. A differencecomponent 71 is similar to the difference component 67 and conducts asignal on its output 66 that corresponds to the value of a signal on itsinput 72 less the value of a signal on its input 73.

The mode transfer circuitry 28 also includes mode select logic circuitryreferred to as 80, which is shown in mode detail in FIG. 9; however, themode select function may be any conventional logic circuitry whichfunctions as herein described. The logic 80 functions to energize itsoutputs 14, 15 and 16, selectively, to connect and disconnect thefeedback controllers 23, 24 and 25 to the summing devices 12 and 13 inaccordance with the desired mode of operation in response to theoperation of the mode select pushbutton CPB, TPB or BPB. Also, the modeselect logic 80 functions to energize its output 64 when the plant is inthe turbine-follow mode, its output 81 when the plant is in theboiler-follow mode, and its output 46 when the plant is in thecoordinated mode. The remaining components of the plant transfercircuitry 28 together with a detailed description of the method andsystem of the present invention will be explained in detail inconnection with its operation prior to, during and subsequent to atransfer between turbine-follow, boiler-follow and coordinated controlmodes.

Assuming that the boiler-turbine power plant is operating in theturbine-follow mode, the mode select logic 80 is in a condition wherethe contact 17 in the output 20 of the feedback controller 23 is closed;and the contacts 18 and 19 in the outputs 21 and 22, respectively, ofthe controllers 24mm 25 are open. Thus, the feedforward signal on theoutput 11 of the load demand computer is input through the summingdevice 13 and the manual/automatic transfer station 35 to the boilermaster control 38 without correction. However, thefeedforward signal onthe output 11 to the summing device 12 is trimmed by a feedbackerror'signal on the output 20 of the feedback controller 23 to eitherraise or lower the steam inlet valve'position in accordance with thedifference between the actual throttle pressure and the throttlepressure set point. Therefore. the firing rate of the boiler isdetermined solely in accordance with the value of the feedforward signalon the output 11; and any deviation in the throttle pressure set pointis corrected by the raising or lowering of the steam inlet valves. Atthe same time, the output 64 of the mode select logic 80 is energizedwhichresults in the track enable input 45 being energizedand'a zerosignal being selected by the signal selector 53 so that the output online 42 is at zero potential, and the'controller 24 is driven to havezero potential at its output. The input 45 to the controller 24 isenergized by a circuit which includes line 64, OR gate 85, line 45 andthe controller 24. Also, the controller 25 is driven to zero potentialby a circuit which includes the deenergized output 46 from the logic 80,negative function 86, OR gate '87 arid the track enable input 46; and byanother circuit which includes OR gate 90, negative function 92 andinput line 75 to the signal selector 54. A typical situation may existwith the" plant operating in the turbine-follow mode wherein the valueofthe signal on the output 11 of the load demand computer 10-is at 50percent load for example, and the error signal on line 20 from thefeedback controller 23 may be at +10 percent for example. This conditionresults in a signal of 60 percent at the output 31 of the summing device12 for input'to the manual/automatic transfer station 32 and to theturbine master control 33 over line 39. The feedforward signal of 50percent is also conducted to the summing device 13, which is uncorrectedby the feedback controller 24 over line 34 to the manual/automaticstation 35. In this condition the boiler master control is receiving asignal which corresponds to a 50 percent load while the turbinecontrolis receiving a signal which corresponds to a 60 percent load inorder to keep the system properly balanced.

The plant can be transferred between operating modes either manually orautomatically in response-to a predetermined contingency. The plantsystem, of course, in actual practice, is much more sophisticated in itsstructure on function; and for the sake of clarity in describing theinventions, only the function necessary to describe the presentembodiment of the invenillustrated. .1 I,

To transfer from the turbine-followmode, previously tion undermanualtransfer conditions is described and described, to the boiler followmodejthe pushbutton BPB is depressed which activates a pulse generator93 by a circuit which extends from BPB, and includes an OR gate 94. Theactivation'ofthe pulse generator93 produces a pulse on output 37, whichin'one'practical embodiment of the invention is of 5 second duration; vThe entire transfer occurs during the existence of this 5 second pulse.However, it is to be understood that the time, such as in the order of 1second, or shorter, for example. In response to the occurrence the pulseon line 37 from the output of thepulse generator 93, the line 27 to'theload demand computer 10 is energized which enables the load demandcomputer to accept and track the output signal'present on the input 30.The manual transfer stations 32 and 35 in response to the energizing ofthe input 37 prevent any change in the value of the signal on lines 38and 40 to the input of the turbine master control 33 and the boilermaster control 36. v

The occurrence on the pulse on line 37 also energizes inputs95 96 and 97to AND gates 98, 99 and 100 respectively. The AND gate 98 conducts inresponseto the occurrence of such pulse from the generator 93 becausethe other input 101 to the AND gate 98 is energized by the operation ofthe pushbutton BPB as shown in FIG. 1. The conductingof the AND gate 98applies a digital signal on line 56 to the input of the signal selector51, which selects the analog signal from the input 50- to be tracked bythe load demand computer 10.

Also, the operation of the pushbutton BPB energizes an input 102 to themode select logic 80 which deenergizes its output line 14 to open thecontact 17 of the controller 23 to disconnect the feedback circuit fromthe summing device 12; and energizes its output line 15 to close thecontact 18 in the output of the controller 24 to connect the feedbackloop to the boiler master control 36 through the summer 13 andthemanual- /automatic station 35. While the input 102 of the mode selectlogic 80 is energized, all of the outputs 64, 81 and 46 of the modeselect logic 80 are deenergized to represent that the plant is not inany one of its operating modes, but being transferred between'modes. Thedeenergizing of the output line 64 from the mode select logic 80 removesthe select zero signal command'from the signal selector 5,3; and thedeenergizing of the line 81 removes the select zero command over input60 to the signal selector 52. The controller 25 remains in the samecondition with respect to its input 46 and 43 as it was in the turbinefollow mode.

To summarize the operation of the transfer system to this point in thedescription, the operation of the pushbutton BPB and the occurrence ofthe pulse at the output of the pulse generator 93 has governed theautomatic/manual transfer station 32 and 35 to hold the signals onoutput 38 and 40 to their pre-transfer value; has operated the loaddemand computer 10 to track the pretransfer signal on line 38 governingthe turbine master control 33; and has disconnected and connected thefeedback controllers 23 and 24 respectively. Also, the controller 24 istracking the signal on input line 66 of the selector 53 from thedifference function device '71. The device 71 is conducting a signalequal in value to the pre-transfer value of the signal to the boilermaster control 36'less the value of the modified feedforward signalwhich was tracked prior to transfer.

Assuming the same quantitative situation to exist as previouslydescribed; that is, the trimmed feedforward signal to the turbine mastercontrol 53 in the turbine follow mode prior to transfer was equivalentto a 60 percent load, and the uncorrected feedforward signal to theboiler master control 36 was equivalent to a 50 percent'load thefeedforward signal on line 11 during vice 71 is equal to lO percent,which is conducted to the input 44 of the feedback controller 24, whichdrives the feedback controller 24 to produce a l percent signal at itsoutput 21, which is input to the summing device 13. Thus, the value ofthe signal at the output 34 of the summing device 13 is equal to 50percent which is the same value present previous to the beginning of thetransfer; and the output signal on line 31 of device 12, to the turbinemaster control equal to a 60 percent load signal, which is thepre-transfer value of such signal. At the termination of the secondpulse from the generator 93, the transfer is deemed complete; and thesystem is placed in the following condition.

A negative function 106 to the input of the mode select logic 80 causesthe output line 81 of the mode select logic 80 to be energized which isrepresentative of the plant being in the boiler-follow mode. The signalon the output line 81 causes the signal selector 52 to conduct a zerosignal on its output 41 to drive the signal on the output 20 of thecontroller 23 to zero in preparation for the next transfer. Also, ANDgate 83 ceases to conduct which deenergizes the track enable input 45 ofthe controller 44 so that the boiler feedback controller 44 now respondsto an error signal corresponding to the actual throttle pressure and theset point throttle pressure for the boiler-follow mode.

To transfer to the coordinated mode of operation the pulse generator 93is activated in response to the operation of the pushbutton CPB, togenerate the five second pulse to hold the input signal to the controlmechanisms 33 and 38 at their pre-transfer value in the same manner asdescribed in connection with the transfer from the turbine-follow to theboilerfollow mode. The mode select logic 80 is energized through input107, which energizes the output line 14, deenergizes the output line 15,and energizes the output line 16 so that contacts 17 and 19 are closedand contact 18 is opened in the outputs 20, 21 and 22 respectively ofthe feedback controllers 23, 24 and 25 to connect both the megawatt andturbine throttle pressure feedback loops to the summing devices 13 and12 respectively.

During transfer to the coordinated mode, the track enable input 27 tothe LDC computer 10, the track enable input 44 to the feedbackcontroller 23, and the track enable input 45 to the feedback controller25 is energized. The circuit for energizing the track enable input 27includes line 37 at the output of the pulse generator 93. The circuitfor energizing the track enable input 44 includes the output of the ANDgate 100 and the OR gate 110. The circuit for energizing the trackenable input 46 includes the output of the AND gate 100, line 108, andOR gate 87.

The load demand computer during this transfer tracks the analog signalon input line 57, which signal is connected to the megawatt detector MWand is equal in value to the actual megawatt output of the turbine. Thepresence of a signal on line 56C at the output of the AND gate 100selects the signal on the input 57. The controller 23 tracks the analogsignal occurring on input 63 to the selector 52 from the differencedevice 57 during transfer to the coordinated mode. The circuit forselecting the input 63 includes the output of the AND gate 100 and theinput 61 to the selector 52. Also, during the transfer to thecoordinated mode the controller 25 tracks the analog signal on the input77 to the signal selector 54. The circuit for selecting the signal onthe input 77 includes the output of the AND gate 12 100, line 108, theOR gate 90, and the input 76 to the signal selector 54.

Assuming that in the boiler-follow mode, the value of the signal at theinput to the boiler master control previous to the transfer isequivalent to a 60 percent load demand; and assuming that the value ofthe feedforward signal to the turbine master control 33 previous to thetransfer is equivalent to a 50 percent load demand, then the same signalvalue on each of the inputs to the turbine and boiler master controlmust be 50 and 60 percent after the transfer. In the present example ofthe transfer to the coordinated mode, it is assumed that the actualmegawatt signal value as detected by the detector MW is equivalent to apercent load. Therefore, during the transfer, the value of the signal onthe output 11 of the lload demand computer 10 becomes equal to theactual megawatt signal value which is 70 percent. The difference device67 operates to subtract the 70 percent megawatt signal from the 50percent uncorrected pre-transfer feedforward signal resulting in a -20percent signal on the input 63, which is tracked by the feedbackcontroller 23. Thus, the summing device 12 algebraically adds the 70percent megawatt signal on the output 11 and the 20 percent signal onthe output 20 of the controller 23 resulting in a 50 percent load signalon the input 31 to the manual/automatic transfer station 31, which isidentical to the value of the pre-transfer signal. The pre-transfervalue of the signal to the boiler master control 36 is 60 percent, andthe actual megawatt output signal on line 1 1 is 70 percent. Therefore,the difference device 71 subtracts the 70 percent signal which is inputon line 73 from the 60 percent signal which is input on line 72 to thedifference device 71. Thus, the value of the signal that is tracked bythe controller 25 is equal to 1O percent. The summing device 13 duringthis transfer has a 70 percent signal input from the line 11 and a l0percent signal from the output 22 of the controller 25, which results ina signal having a 60 percent value on the. input 34 to themanual/automatic transfer station 35. Therefore, the pre-transfer valueof the input signal to the boiler master control 36 is identical to thevalue of the signal on the line 34 which is summed by the sum- 'mingdevice 13 during the transfer.

At the end of the five second period, the output'pulse from thegenerator 93 ceases which releases the hold on the manual/automaticstation 32 and 35. The gate ceases to conduct which removes the signalfrom the track enable input to the load demandc'om'puter 10, thefeedback controller 23, and the feedback controller 25. Thus, the loaddemand computer 10 may be operated through its remote control device 58or its manual control device 26 for varying the value of the feedforwardsignal on the output 11; the feedback controller 23 is now generating anerror signal corresponding to the value between the throttle pressureset point and the actual throttle pressure", and the feedback controller25 is generating an error signal corresponding to the difference betweenthe feedforward signalon the output 1 1 and the actual megawatt outputof the plant.

To transfer from the coordinated mode to the boilerfollow mode, thepushbutton BPB is operated to generate the 5 second pulse, as describedin connection with the other transfers. Also, the feedback controller 25and the feedback controller 23 are disconnected from the summing devices12 and 13 while the feedback controller is connected to the summingdevice 13 by the energizing and deenergizing of the appropriate out- 13puts 14, 15 and 16 of the logic 80 as previously described. Assumingthat prior to the transfer the signal that is input to the turbinemaster control 33 is 50 percent, and the signal input to the boilermaster control 36 is 60 percent, such signals are held at theirpretransfer value in response to the presence of the sec ond pulse inthe same manner as previously described. The input 56 to the signalselector 61 is energized by the previously described circuit whichgoverns the load demand computer to track the signal on its input 50which corresponds to the pre-transfer value of the demand signal to theturbine master control 33 which is in the present example 50 percent.Therefore, the signal on the output 11 is changed to be equal to a 50%signal for controlling the turbine master control 33 at the terminationof the transfer in the same manner as described in connection with theprevious examples. The boiler feedback controller 24 is governed totrack an output equivalent to 10 percent which is the tracked signal of50 percent on the output 11 subtracted from the pre-transfer signal of60 percent on the input 72 of the difference amplifier 71. Thus, the 50percent signal and the 10 percent signal are added by the summing device13 to provide a signal equivalent to a 60 percent demand which is thesame value of the demand signal present previous to the transfer. Thedisconnected controllers 23 and are driven to their zero condition atthe termination of the transfer by the previously described circuits.

FIG. 3 schematically shows the primary functions of the load demandcomputer insofar as they affect the method and system of the presentinvention. For example, the energizing of the track enable input 27conditions AND gates 115 and 1 16 to operate a conventional up/downcounter 117, which in turn operates a digitalto-analog converter 118 togenerate the analog signal on the output 11. The selected track signalis input to comparators 120 and 121 which is compared to the signal onthe output 11 for either operating the up/down counter 117 to eitherincrease or decrease the value of the signal. The manual control device26 operates the up/down counter 117 directly through OR gates 122 and123.

FIG. 4 illustrates schematically the track signal selector 51 whichselects either the actual megawatts, the pre-transfer value of theboiler demand signal or the pre-transfer value of the turbine demandsignal to be conducted over its output in response to the energizing ofeither its input for entering the coordinated, turbine-follow orboiler-follow mode of operation. For example, the energizing of theinput 56C closes contacts 130 and 131 of a relay 132 to conduct thesignal on input 57 to the output line 30. The signal selectors 52, 53and 54 may have the same type of an arrangement as the signal selector51.

FIG. 5 illustrates schematically a feedback controller, such as thecontroller 23. When the track enable input 44 is energized, a relay 140is closed which conducts the analog signal from the line 41 through thecontroller circuitry to the output line 20. When the input line 44 isdeenergized, a relay 141 is energized which conducts the error signalbetween the actual throttle pressure and the set point throttle pressureto the output 20. Regardless of the period for integrating the timeconstant, the controller output is immediate when the track enable inputis energized. Although the other controllers differ with respect to theintegration of their time constant, they are all assumed to be similarlyconstructed. For example, the turbine throttle pressure controller 23 inone practical embodiment integrates its time constant in 15 to 20seconds, for example. The megawatt controller 25 integrates its timeconstant in 5 minutes, for example; and the boiler throttle pressurecontroller 24 integrates its time constant in approximately 3 minutes,for example.

FIG. 6 is a schematic arrangement of a conventional summing device, suchas the summing device 12 wherein the analog signals on line 11 and 20are summed to provide the output on lines 31.

FIG. 7 is a typical difference device, such as 67, for example, whereinthe analog input on line is subtracted from the analog signal on line 68to generate the difference on the output 63 in a conventional manner.

FIG. 8 is a manual/automatic transfer station, such as 32, where aninput signal on the line 37 holds the output signal on line 38, forexample, by inhibiting the operation of an up/down counter 150. When theline 37 is deenergized, a signal on the input 31 operates the up/- downcounter 150, which operates a digital-to-analog converter 151 to providethe proper analog signal on the output 38. The manual/automatic stationincludes various logic in clocks to perform other functions includingswitching over from automatic to manual control of the turbine or theboiler, as the case may be, by the operation of the panel buttonsreferred to at 152.

FIG. 9 illustrates schematically one form of the mode select logiccircuitry, which includes the various logic gates to energize thevarious outputs in response to the operation of the pushbuttons BPB, TBPand CBP in the manner described in connection with the operation of thesystem. Flip-flop circuits 160, 161 and 162 permit the operator tomomentarily depress the pushbutton to initiate transfer.

Although a description of the system has been described in connectionwith a transfer from the turbinefollow to boiler-follow mode, from theboiler-follow to the coordinated mode, and from the coordinated mode tothe boiler-follow mode, such description renders opposed transferbetween the other of the modes.

To summarize the operation of the system and method of the presentembodiment of the invention, the initiation of a transfer generates apulse of 5 second duration from the output of the generator 93. Thepresence of the pulse prevents an increase or a decrease in the outputsof the transfer stations 32 and 35 to the input of the boiler andturbine control apparatus 36 and 33. The feedback controllers 23, 24,and 25 are connected and disconnected by the mode select logic dependingon the mode that the system is entering. The track enable input to thecomputer 10 is energized; and the track enable input to the feedbackcontrollers which are to be placed in service upon termination of thetransfer are energized. The feedforward signal from the load demandcomputer 10 is increased or decreased to equal the analog signalselected by the signal selector 51. This signal is equal to thepre-transfer or held output of the manual/automatic station which willreceive an uncorrected feedforward signal at termination of thetransfer. When transferring to the coordinated mode, the signal is equalto the actual megawatt output of the plant.

The feedback controllers 23, 24 or 25, depending on the mode the systemis entering, tracks a selected signal input to its associated signalselector 52, 53, and 54. This tracked signal is equal to the held outputon its associated transfer station 32 and 35 less the tracked signalgenerated by the computer 10. The tracked output signal from thefeedback controller is summed with the tracked signal from the computer10 by its associated summing device 12 and 13.

At the end of the second period the pulse from generator 93 terminateswhich permits the manual- [automatic stations 32 and 35 to respond tovariations in their inputs 31 and 34, and the tracking function of theinservice feedback controller is removed.

It is understood that the inventive features of the system and method ofthe present invention may be implemented with various types of apparatusand components, and the particular circuit is illustrative only.Variations within the spirit and scope of the invention may be made bythose skilled in the art.

What is claimed is:

1. A system for transferring a boiler turbine power plant from oneoperating mode to another rapidly without disturbing the plant processwherein predetermined selected in service feedback loops trim afeedforward signal for controlling the boiler and turbine in eachoperating mode and at least one feedback loop is put in service forchanging operating modes, said system comprising a turbine controlapparatus for controlling turbine inlet valve position in accordancewith the value of an input signal, a boiler control apparatus forcontrolling boiler firing rate in accordance with the value of an inputsignal,

means to generate a feedforward signal representative of desired plantoperation connected electrically to input the feedforward signal to theboiler and turbine control apparatus in parallel,

at least one feedback loop for each control apparatus to generate anoutput signal,

first circuit means including each feedback loop to modify the value ofthe input signal to the boiler and turbine apparatus in accordance withthe output signal value of its associated feedback loop that is inservice for a distinct operating mode,

mode transfer selection means effective when activated to govern theturbine and boiler control apparatus to be unresponsive to a change inthe value of either a feedforward or feedback signal to prevent anychange in turbine valve position and boiler firing rate during a modetransfer,

second circuit means including the feedforward signal generating meansgoverned by the activation of the transfer means to modify the generatedfeedforward signal to be representative of a selected value inaccordance with the operating mode selected by the mode transfer means,

third circuit means governed by the activated transfer means to controleach feedback loop put in service in accordance with the selectedoperating mode to generate a feedback output signal for the firstcircuit means having a value to modify the input signal for the turbineand boiler control apparatus to be representative of the value of theturbine and boiler control apparatus input signal upon activation of thetransfer means, and

means deactivating the transfer means to render the boiler and turbinecontrol apparatus responsive to their respective input signal from thesignal generating means as modified by the first circuit means.

2. A system according to claim 1 wherein the selected value of themodified feedforward signal by the 16 second circuit means isrepresentative of the actual power output of the plant while thetransfer selection means is activated.

3. A system for transferring from one operating mode to anotheraccording to claim 1 wherein at least one feedback loop is taken out ofservice and another feedback loop is put in service to change operatingmodes, and wherein the selected value of the modified feedforward signalis representative of the value of the input signal to the controlapparatus having its associated feedback loop being taken out of serviceby the mode transfer selection means.

4. A system according to claim 1 wherein the first circuit meansincludes a summing device for algebraically adding the feedforwardsignal and the output of the in service feedback loop.

5. A system according to claim 1 further comprising an analog signalgenerating device for each control apparatus responsive to the signalfrom the first and second circuit means to govern the value of the inputsignal for its associated control apparatus, and each said deviceincludes means responsive to the activation of the transfer selectionmeans to hold the output of the analog signal generating devicestationary while the transfer selection means is activated.

6. A system according to claim 1 wherein the transfer selection meansand the transfer deactivating means includes a pulse generator forgenerating a selected pulse of a predetermined time duration, and thetransfer selection means is activated during the time duration of thegenerated pulse only.

7. A system according to claim 6 wherein the duration of the generatedpulse does not exceed 5 seconds.

8. A system according to claim 1 wherein each feedback loop includes aproportional plus integral controller, and the third circuit meansincludes a difference function device for each control apparatusgoverned by input signals representing the value of the input signal ofits associated control apparatus upon activation of l the transferselection means and the value of the feedforward signal modified by thesecond circuit means to conduct a difference signal, and the controlleris governed by the value of the difference signal while the transferselection means is activated to generate the value of such differencesignal at its output.

9. A system for transferring from one operating mode to another rapidlyand without disturbing the plant process of a boiler-turbine powerplant, comprising a load demand signal generator operative to generatean output signal having a value corresponding to a desired plant load,said device having means to increase and decrease the output signal inaccordance with the value of a selected tracking signal,

a turbine control apparatus and a boiler control apparatus, eachoperative to govern the operation of the boiler and the turbinerespectively in accordance with the value of an input signal,

a throttle pressure feedback controller for each control apparatusoperative to generate an error signal representative of the requiredchange in the desired load signal for its associated apparatus tooperate its respective turbine and boiler to maintain a predeterminedthrottle pressure, each said controller also having means to generate anerror signal in accordance with the value of a selected tracking signal,

a summing device for each control apparatus connected at one input toreceive the error signal from 17 its associated feedback Controller andconnected at anotherinput to receive the desired load signal from thegenerator, each said device being operative to generate an output signalcorresponding to the algebraic sum of the value of the input signals,

an analog signal generator for each control apparatus connected at itsinput to theoutput signal of its associated summing device, said signalgenerator being operatively connected to generate a demand signal forits associated apparatus in accordance with the value of the signal fromthe summing device, each said device also including a holding means whenactivated to prevent the output signal to its associated controlapparatus from responding to a change in its input signal,

a difference function device for each control apparatus having one inputconnected electrically between the output of its associated summingdevice and another input connected to the output of the load demandgenerator, each said device being operative to generate at its output asignal representative of the value of the signal at its one input lessthe value of the signal on its other input,

transfer means operative when activated to initiate a transfer to aselected operating mode,

means responsive to the activation of the transfer means to activate theholding means of each analog signal generator,

means governed by the activation of the transfer means to connect anddisconnect selected feedback controllers to and from their respectivesumming devices in accordance with the selected mode,

means governed by the activation of the transfer means to increase ordecrease the desired load signal at the input to each summing device totrack the value of the signal at the output of the analog signalgenerator for the control apparatus disconnected from the output of afeedback controller upon deactivation of the transfer means,

means governed by the activation of the transfer means to operate thefeedback controller to be connected to its summing device in accordancewith a selected mode to track the signal at the output of the differencedevice, and

means deactivating the transfer means, whereby the signal to the inputof each analog device upon deactivation of the transfer means isrepresentative of the input signal upon activation of the transfermeans.

10. A system according to claim 9 further comprising a megawatt feedbackcontroller for the boiler control apparatus operative to generate anerror signal representative of the required change in the load signalfor the boiler control apparatus to operate the boiler control apparatusto govern the turbine to maintain a predetermined power output, saidfeedback controller also having means to generate an error signal inaccordance with the value of a selected tracking signal,

means governed by the activation of the transfer means to connect thethrottle pressure feedback controller to the summing device associatedwith the turbine control apparatus and the megawatt feedback loop to thesumming device associated with the boiler control apparatus as theselected mode,

means governed by the activation of the transfer means for the selectedmode to increase or decrease the 'desired loadsignal in accordance withthe value of the actual power output of the plant, and means governed bythe activation of the transfer means for the selected mode to operatethe connected throttle pressure controller and the megawatt pressurecontroller to track the signal at the output of its associateddifference device. 11. A method of transferring rapidly from one toanother of at least two modes of operation in a boiler-turbine powerplant without disturbing the plant process wherein a control signalrepresentative of desired plant load is generated to control in parallelthe boiler portion of the plant and the steam turbine portion of theplant in both of the modes and a feedback loop modifies the controlsignal to the boiler only in one of the modes and a feedback loopmodifies the control signal to the turbine only in the other mode, saidmethod comprising holding the load demand signals to both the boiler andturbine at their pre-transfer value to prevent a change in their valueduring the transfer,

changing the connection of the feedback loops from one of the portionsof the plant to the other,

changing the desired load signal to be representative of the value ofthe pre-transfer modified demand signal for one of the portions of theplant generating a feedback signal for the other portion representativeof the value of the pre-transfer load demand signal to said portion lessthe value of the desired load signal, and

releasing the hold on the load demand signals to both the boiler andturbine,

whereby the load demand signal to both the boiler and turbine is thesame before and after the changing of the feedback loops from one to theother of the elements.

12. A method of transferring from one operating mode to another rapidlyand without disturbing the plant process of a boiler turbine powerplant, wherein selected feedback loops trim a feedforward signal toprovide a load demand signal for controlling the boiler and turbine ineach operating mode, said method comprising holding the load demandsignal to the boiler and turbine at a constant value during thetransfer, changing the feedforward signal to be representative of aselected value,

generating a feedback signal having a value, which when summed thefeedforward signal is representative of the value of the load demandsignals held during the transfer, summing the generated feedback signalswith the changed feedforward signal for the loops to be put in serviceupon termination of the transfer, and

releasing the hold on the load demand signals to terminate the transfer.

13. A method according to claim 12 wherein the changed feedforwardsignal is representative of the actual power output of the plant attimes when the transfer effects the trimming of the load demand signalto both the turbine and boiler.

14. A method according to claim 12 wherein the changed feedforwardsignal is representative of the pretransfer trimmed load demand signalat times when the transfer effects the trimming of the other of theboiler turbine load demand signals only.

19 15. A system for transferring from one operating mode to anotherrapidly and without disturbing the plant process of a boiler turbinepower plant wherein selected connected feedback loops trim a feedforwardsignal to provide a load demand signal for controlling I the boiler andturbine in each operating mode; said sys tem comprising means toinitiate a transfer from one mode to another, means responsive to theinitiation of a transfer to hold the load demand signals to the boilerand turbine at a constant value during the transfer,

at termination of the transfer.

1. A system for transferring a boiler turbine power plant from oneoperating mode to another rapidly without disturbing the plant processwherein predetermined selected in service feedback loops trim afeedforward signal for controlling the boiler and turbine in eachoperating mode and at least one feedback loop is put in service forchanging operating modes, said system comprising a turbine controlapparatus for controlling turbine inlet valve position in accordancewith the value of an input signal, a boiler control apparatus forcontrolling boiler firing rate in accordance with the value of an inputsignal, means to generate a feedforward signal representative of desiredplant operation connected electrically to input the feedforward signalto the boiler and turbine control apparatus in parallel, at least onefeedback loop for each control apparatus to generate an output signal,first circuit means including each feedback loop to modify the value ofthe input signal to the boiler and turbine apparatus in accordance withthe output signal value of its associated feedback loop that is inservice for a distinct operating mode, mode transfer selection meanseffective when activated to govern the turbine and boiler controlapparatus to be unresponsive to a change in the value of either afeedforward or feedback signal to prevent any change in turbine valveposition and boiler firing rate during a mode transfer, second circuitmeans including the feedforward signal generating means governed by theactivation of the transfer means to modify the generated feedforwardsignal to be representative of a selected value in accordance with theoperating mode selected by the mode transfer means, third circuit meansgoverned by the activated transfer means to control each feedback loopput in service in accordance with the selected operating mode togenerate a feedback output signal for the first circuit means having avalue to modify the input signal for the turbine and boiler controlapparatus to be representative of the value of the turbine and boilercontrol apparatus input signal upon activation of the transfer means,and means deactivating the transfer means to render the boiler andturbine control apparatus responsive to their respective input signalfrom the signal generating means as modified by the first circuit means.2. A system according to claim 1 wherein the selected value of themodified feedforward signal by the second circuit means isrepresentative of the actual power output of the plant while thetransfer selection means is activated.
 3. A system for transferring fromone operating mode to another according to claim 1 wherein at least onefeedback loop is taken out of service and another feedback loop is putin service to change operating modes, and wherein the selected value ofthe modified feedforward signal is representative of the value of theinput signal to the control apparatus having its associated feedbackloop being taken out of service by the mode transfer selection meanS. 4.A system according to claim 1 wherein the first circuit means includes asumming device for algebraically adding the feedforward signal and theoutput of the in service feedback loop.
 5. A system according to claim 1further comprising an analog signal generating device for each controlapparatus responsive to the signal from the first and second circuitmeans to govern the value of the input signal for its associated controlapparatus, and each said device includes means responsive to theactivation of the transfer selection means to hold the output of theanalog signal generating device stationary while the transfer selectionmeans is activated.
 6. A system according to claim 1 wherein thetransfer selection means and the transfer deactivating means includes apulse generator for generating a selected pulse of a predetermined timeduration, and the transfer selection means is activated during the timeduration of the generated pulse only.
 7. A system according to claim 6wherein the duration of the generated pulse does not exceed 5 seconds.8. A system according to claim 1 wherein each feedback loop includes aproportional plus integral controller, and the third circuit meansincludes a difference function device for each control apparatusgoverned by input signals representing the value of the input signal ofits associated control apparatus upon activation of the transferselection means and the value of the feedforward signal modified by thesecond circuit means to conduct a difference signal, and the controlleris governed by the value of the difference signal while the transferselection means is activated to generate the value of such differencesignal at its output.
 9. A system for transferring from one operatingmode to another rapidly and without disturbing the plant process of aboiler-turbine power plant, comprising a load demand signal generatoroperative to generate an output signal having a value corresponding to adesired plant load, said device having means to increase and decreasethe output signal in accordance with the value of a selected trackingsignal, a turbine control apparatus and a boiler control apparatus, eachoperative to govern the operation of the boiler and the turbinerespectively in accordance with the value of an input signal, a throttlepressure feedback controller for each control apparatus operative togenerate an error signal representative of the required change in thedesired load signal for its associated apparatus to operate itsrespective turbine and boiler to maintain a predetermined throttlepressure, each said controller also having means to generate an errorsignal in accordance with the value of a selected tracking signal, asumming device for each control apparatus connected at one input toreceive the error signal from its associated feedback controller andconnected at another input to receive the desired load signal from thegenerator, each said device being operative to generate an output signalcorresponding to the algebraic sum of the value of the input signals, ananalog signal generator for each control apparatus connected at itsinput to the output signal of its associated summing device, said signalgenerator being operatively connected to generate a demand signal forits associated apparatus in accordance with the value of the signal fromthe summing device, each said device also including a holding means whenactivated to prevent the output signal to its associated controlapparatus from responding to a change in its input signal, a differencefunction device for each control apparatus having one input connectedelectrically between the output of its associated summing device andanother input connected to the output of the load demand generator, eachsaid device being operative to generate at its output a signalrepresentative of the value of the signal at its one input less thevalue of the signal on its other input, transfer means operative whenactivated to initiate a transfer to a selected operating mode, meansresponsive to the activation of the transfer means to activate theholding means of each analog signal generator, means governed by theactivation of the transfer means to connect and disconnect selectedfeedback controllers to and from their respective summing devices inaccordance with the selected mode, means governed by the activation ofthe transfer means to increase or decrease the desired load signal atthe input to each summing device to track the value of the signal at theoutput of the analog signal generator for the control apparatusdisconnected from the output of a feedback controller upon deactivationof the transfer means, means governed by the activation of the transfermeans to operate the feedback controller to be connected to its summingdevice in accordance with a selected mode to track the signal at theoutput of the difference device, and means deactivating the transfermeans, whereby the signal to the input of each analog device upondeactivation of the transfer means is representative of the input signalupon activation of the transfer means.
 10. A system according to claim 9further comprising a megawatt feedback controller for the boiler controlapparatus operative to generate an error signal representative of therequired change in the load signal for the boiler control apparatus tooperate the boiler control apparatus to govern the turbine to maintain apredetermined power output, said feedback controller also having meansto generate an error signal in accordance with the value of a selectedtracking signal, means governed by the activation of the transfer meansto connect the throttle pressure feedback controller to the summingdevice associated with the turbine control apparatus and the megawattfeedback loop to the summing device associated with the boiler controlapparatus as the selected mode, means governed by the activation of thetransfer means for the selected mode to increase or decrease the desiredload signal in accordance with the value of the actual power output ofthe plant, and means governed by the activation of the transfer meansfor the selected mode to operate the connected throttle pressurecontroller and the megawatt pressure controller to track the signal atthe output of its associated difference device.
 11. A method oftransferring rapidly from one to another of at least two modes ofoperation in a boiler-turbine power plant without disturbing the plantprocess wherein a control signal representative of desired plant load isgenerated to control in parallel the boiler portion of the plant and thesteam turbine portion of the plant in both of the modes and a feedbackloop modifies the control signal to the boiler only in one of the modesand a feedback loop modifies the control signal to the turbine only inthe other mode, said method comprising holding the load demand signalsto both the boiler and turbine at their pre-transfer value to prevent achange in their value during the transfer, changing the connection ofthe feedback loops from one of the portions of the plant to the other,changing the desired load signal to be representative of the value ofthe pre-transfer modified demand signal for one of the portions of theplant generating a feedback signal for the other portion representativeof the value of the pre-transfer load demand signal to said portion lessthe value of the desired load signal, and releasing the hold on the loaddemand signals to both the boiler and turbine, whereby the load demandsignal to both the boiler and turbine is the same before and after thechanging of the feedback loops from one to the other of the elements.12. A method of transferring from one operating mode to another rapidlyand without disturbing the plant process of a boiler turbine powerplant, wherein selected feedback loops trim a feedforward signal toprovide a load demand signal for controlling the boiler and turbine ineach operating mode, said method comprising holding the load demandsignal to the boiler and turbine at a constant value during thetransfer, changing the feedforward signal to be representative of aselected value, generating a feedback signal having a value, which whensummed the feedforward signal is representative of the value of the loaddemand signals held during the transfer, summing the generated feedbacksignals with the changed feedforward signal for the loops to be put inservice upon termination of the transfer, and releasing the hold on theload demand signals to terminate the transfer.
 13. A method according toclaim 12 wherein the changed feedforward signal is representative of theactual power output of the plant at times when the transfer effects thetrimming of the load demand signal to both the turbine and boiler.
 14. Amethod according to claim 12 wherein the changed feedforward signal isrepresentative of the pre-transfer trimmed load demand signal at timeswhen the transfer effects the trimming of the other of the boilerturbine load demand signals only.
 15. A system for transferring from oneoperating mode to another rapidly and without disturbing the plantprocess of a boiler turbine power plant wherein selected connectedfeedback loops trim a feedforward signal to provide a load demand signalfor controlling the boiler and turbine in each operating mode; saidsystem comprising means to initiate a transfer from one mode to another,means responsive to the initiation of a transfer to hold the load demandsignals to the boiler and turbine at a constant value during thetransfer, means governed by the initiation of a transfer to change thefeedforward signal to be representative of a selected value, means togenerate another signal for each feedback loop to be connected inaccordance with the selected operating mode, said signal having a valuewhich when summed with the changed feedforward signal is representativeof the value of the load demand signal held at the constant value, meansto sum the feedforward signal and the other feedback generated signalfor the selected connected loops, and means to release the hold on theload demand signal at termination of the transfer.